The present invention relates to a hot cathode and its production process. Such cathodes are used in electron tubes, tubes with localized parameters such as triodes and tetrodes or tubes with distributed parameters such as klystrons and magnetrons used at ultra-high frequencies. The invention also relates to such electron tubes.
The power developed by electron tubes at ultra-high frequencies is particularly limited by the current density produced by the cathode.
For many years, hot cathodes have been known which comprise a tungsten matrix impregnated with barium and calcium aluminates in variable proportions. The performances of these cathodes are approximately 1 to 3 A/cm.sup.2, depending on the aluminate compositions used and for temperatures between 1000.degree. and 1035.degree. C. The prior art has proposed various solutions for improving the performances of these cathodes. One of the proposed solutions consists of the surface deposition of a high work output refractory metal such as iridium, osmium, ruthenium or rhenium. The current density gain for the same temperature is approximately a factor of 3. The temperature gain for the same current density is approximately 80.degree. C.
Instead of depositing a film of a refractory metal with a high work function on the cathode surface a more recent solution consists of mixing tungsten powder with said metal in proportions varying between 10 and 80% and then impregnating the cathode. The electron emission characteristics of these cathodes are 2 to 5 times higher than those of cathodes of tungsten only, depending on the metal used in the mixture. Particular reference should be made in this connection to French Pat. No. 77/18822 published as No. 2,356,263.
It has been possible to compare the performances of these three types of cathodes, i.e. impregnated only, impregnated and covered and mixed impregnated cathodes by means of an experimental study based on a group of curves giving the work function as a function of the temperature for different cathodes.
The curves are in the form of garlands and have a minimum for a temperature close to the optimum temperature corresponding to the optimum covering of the cathode. There is found to be a reduction in the work function of the cathode at temperatures lower than the optimum temperature for curves corresponding to covered impregnated cathodes. There is also a reduction of the minimum of the curve in the case of mixed impregnated cathodes.